Anticaking-agent for ammonium nitrate



cheaply than'by the prior art methods. ticular object is "an improved method of grinding am- United 2,879,133 ANTICAKING-AGENT'FOR AMMON IUM NIT RATE Paul 0. Marti, Jr., Munster, Ind., assignor to Standard Oil Company, Chicago, 11]., a corporation oflndiana No Dra'wing. .Application December 31, 1952 Serial-N0. 329,147

11 Claims. (CL' 23-103) up moisture and the individual particles tend to agglomerate to form lumps; under conditions of high humidity the ammonium nitrate particles tend to form a solid. mass. This agglomeration tendency of ammonium nitrate. in-thespresence of atmospheric humidity is commonly called caking" or settingl For most commercial uses it is desirable that the ammonium. nitrate particles be free-flowing, e. g., when used as fertilizer, it is necessary that the granules flow readily from 'the hopper through the distributing slots onto the ground. For use in the explosives industry the powdered ammonium nitratev should flow readily from the storage bins into the mixing vessel. Bagged ammonium nitrate granules tend to cake and this cake must be broken up before use.

Unless considerable care is taken tomaintain low at- 'mospheric"humidity, it is extremely difiicultfto obtain powdered ammonium nitrate of aparticle size suitable for explosive use. Even with care regarding atmospheric "humidity, it is 'atpr'esent a 'slow and expensive grinding operation to. prepare'finely powdered ammonium nitrate.

It is an object of this invention to prepare a particulate ammonium nitrate composition that is not susceptible to atmospheric humidity. Anotherobject of the invention is a composition consisting of ammonium nitrate particles coated withv a defined anticaking-agent. Still another ob- ;ject is a composition consisting of ammonium nitrate particles coated with a comminuted agent, which composition iseharacterized by essentially 'free-flowability at atmospheric humidity and by a storage stability substantially "that of dynamitegrade ammonium nitrate. A particular ob ect of the invention is a method of grinding ammonium nitrate in the presence of atmospheric humidity to produce powdered ammonium nitrate more'quickly and more Another .par-

monium nitrate in "the presence of atmospheric humidity which consists of grinding ammonium nitrate particles in the presence of a defined anti'cakin'g-agent (grinding aid).

It has been discovered that ammonium nitrate particles coated with comminute'd H 5 treated ir'on-ferrocyanide' are 'e'ssentiallyncn-ea'kin' in the presence of atmospheric humidity. i

The improved method of grinding ammonium nitrate in the presence-of-atmospheric humidity grinds ammonium nitrate particles in the presence of an-e'ffective amount of \said comminute'dagent; preferably using an amount at least sufiicient to substantially coat the surfaceof the ammonium nitrate particles produced in the grinding operation.

The term ammonium nitrate as used in this specifica tion and in the claims is intended to include either ordinary commercial grade ammonium nitrate or dynamite grade ammonium nitrate or pure ammonium nitrate.

The term atmospheric humidity is intended to mean the presence of water vapor in air at various temperatures; or even the atmospheric condition commonly known as fog. More particularly, the term is intended to include those conditions at which the water vapor present in the air would cause agglomeration and caking of 'ammonium nitrateparticles.

The anticaking-agent of this invention is obtained by treating an iron-'ferrocyanide with hydrogen sulfide, H s. The term iron-ferrocyanidef -is intended to include ferric ferrocyanide, ferrous fe'rroc'yanide and the complex materials known as the Prussian blues. The Prussian blues are divided into (a) soluble Prussian blue, and (b) insoluble Prussian blue. These Prussian blues contain in addition to the ironand cyanide ions, alkali metal ions and, often, ammonium ions. (None of the commercial insoluble Prussian blues tested were pure ferric 'ferrocyanide.) The preferred anticaking-agent is insoluble Prussian blue which has been treated with H 5.

The combustion catalyst of this .invention .is obtained by exposing iron-ferro'cy'anide to the action of hydrogen The H 5 is rapidly taken up for a time but the rate of sulfur addition decreases and finally stops. The point of completed sulfur addition is indicated fairly well by the fact that the temperature of the reaction zone reaches a constant level. The H S treated material has a strong odor of hydrogen sulfide ifcooled to room temperature quickly after the interaction has come to an equilibrium, as indicated by a constant reaction zone temperature. The odor is lost slowly on prolonged standing. The amount of H 8 taken up will vary somewhat with the particular material charged. However, 'generally the agent will contain between about 2 and 7 weight percent of sulfurypr'eferably the agent will contain between about 4 and '5 weight'percent of sulfur. The H 8 treatment does not adversely change the particle size of the starting material.

The term H 8 treated-iron ferrocyanide is intended to include the product of the H 5 treatment of ironferrocyanide material Whether or not the producthas a marked H 8 odor or is substantially odorless.

The anticaking-agents (grinding aids) of this invention must have a particle size markedly smaller than the particle size of the ammoniumnitrate which is to be treated. In order to obtain substantially complete coating a comminuted mixture of the agent is preferred. The product of treating the commercially available Prussian blues has a particularly good particle size distribution for this purpose. Especially effective agents have a particle size between about 0.01 and about 10 microns, as much as 10 weight percent of particles having an average size somewhat greater than 10 microns may also be present.

It is preferred to use agents having a particle size mainly atmosphere. The coating slows somewhat, the rate of solution of the nitrate in liquid water; however, the sol ubility in liquid water is not changed.

The anticaking-agents of this invention adhere readily to the ammonium nitrate particles. A non-caking composition is easily obtained by simple mixing of ammonium ammonium nitrate particles and the anticaking-agent may be added simultaneously as separate streams into the feed chamber of the'grinder. When operating with minimum amounts of anticaking-agent (grinding aid), it may be desirable to regrind the product from the first grinding operation.

Some improvement in caking characteristics can be obtained by the addition of minute amounts of agent. For best results and maximum improvement in caking characteristics at least suflicient agent should be added to the ammonium nitrate particles to substantially coat the entire outer surface of the ammonium nitrate particles. Amounts of agent in excess of this quantity have no significant beneficial etfect on the free-flowability of the coated ammonium nitrate particles. The amount of anticaking-agent needed to coat a given quantity of ammonium nitrate particles is dependent upon the size of the ammonium nitrate particles. The smaller the particles the more agent needed per unit weight of ammo nium nitrate. For fertilizer size grains as little as 0.1 weight percent, based on ammonium nitrate, of agent is suflicient. For the ammonium nitrate particles is socalled dynamite grade, between about 0.3 and 0.5 weight percent of agent may be needed. For the fine powder desirable for some explosive usages wherein substantially all of the particles pass through a 325 mesh screen as much as 4 or 5 weight percent of agent may be needed. In general the amount of anticaking-agent needed to obtain a particulate ammonium nitrate composition characterized .by essentially free-flowability at atmospheric humidity is between about 0.1 and 5 weight percent of comminuted anticaking-agent wherein the larger amount corresponds to the finer average particle size of the ammonium nitrate.

The anticaking-agcnts of this invention catalyze the decomposition of ammonium nitrate.

treated insoluble Prussian blue in ammonium nitrate can result in an explosion when the ammonium nitrate is heated to about 130 C. The higher the purity of the ammonium nitrate, the more catalyst required. The pres- .ence of agent markedly increases the rate of decomposition, as measured by gas evolution, at higher atmospheric temperatures. However, in general, the amounts of anticaking-agcnts needed to produce free-flowing ammonium nitrate particles at atmospheric humidity are sulficiently less than the amounts needed to catalyze the decomposition of ammonium nitrate that the coated particles are characterized by storage stability substantially that of dynamite grade ammonium nitrate. Dynamite grade ammonium nitrate contains a sufficient amount of impurities such that the stability is not as great as pure ammonium nitrate; nevertheless, the stability of the dynamite grade is sufiicicnt for all explosive purposes.

In order to illustrate the results obtainable with the anticaking-agents of this invention, the following illustrative examples are set forth:

Example I RlUhIr-ThiS was carried out with Hercules dynamite grade ammonium nitrate; This material was analyzed for particle size distribution using the Rotap screen analysis The presence of H 8 i i '4 method. The analysis was carried out at a room temperature of about 70 F. and a relative humidity of about 30%. The analysis showed the following particle size distribution:

Screen mesh number: Wt. percent retained A quantity of this ammonium nitrate was placed in an 8- ounce screw-top jar. The top was loosely screwed on so that the atmosphere would have access to the interior of the jar. The jar was placed on a shelf in a room; after 7 days exposure to room temperatures between about 70 and F. and relative humidities of between about 30 and 70%, the ammonium nitrate particles had caked not only together, but had adhered to the glass surface. It was possible to invert the jar without anyof the ammonium nitrate particles becoming detached from the mass or the mass becoming detached from the bottom of the 1 Run 2.-This was carried out by mixing about 0.5 weight percent of H 8 treated insoluble Prussian blue, based on ammonium nitrate, with the same amount of ammonium nitrate particles as used in-run 1. The mixing was accomplished by stirring the agent and the ammonium nitrate particles with a spatula."

An electron microscopic examination of the agent showed thatthe particle size ranged from 0.03 y to about 3 microns; most of the particles were less than 0.1 micron 1D. size.

Microscopic examination of the mixture showed that the ammonium nitrate particles were completely coated with a layer of agent. The individual particles showed the green'color of the H S treated insoluble Prussian blue.

The coated particles were stored in a loosely screw capped bottle. The particles flowed quite easily and contained nolumps after 5 months exposure to room temperatures of between 70 and 85 F. and relative humidities between about 20 and 80%. l

The H S treated insoluble Prussian blue was prepared by placing several grams of a commercial grade of insoluble Prussian blue (essentially pure ferric ferrocyanide) having a sulfur content of 0.16 weight percent into a flask. The flask and contents were at room temperature, about 20 C. Free H 5 gas was slowly passed through the finely powdered mass of insoluble Prussian blue. The temperature of the reaction zone as measured by a thermometer inserted into the powder immediately increasedand continued to. increase fairly rapidly to a temperature of about 60 C. The temperature increase slowed downat thispoint and slowly rose to about 65 C. and became substantially constant at that point. The H S gas stream was turned off when the reaction zone temperature leveled out. The flask and its contents were cooled to room temperature. The blue color of the insoluble Prussian blue had changedto a dark green color. The powderwas analyzed and was found to contain 4.26 weight percent of sulfur. The H S treated-insoluble Prussian blue had a very strong odor of H 8 at room temperature even after many hours standing in an open beaker. I

A satisfactory agent can be made by slurrying the iron-ferrocyanide in water orhydrocarbon and passing the hydrogen sulfide through the slurry until either the desired sulfur content has been reached or the sulfur addition has been completed.

Example ll Run 3.-Dynamite grade ammonium nitrate was ground in a Mikro-Pulverizer at 'a speed of 9600 rpm. using a slotted screen at a relative humidity of 70%. The first ground material was reground under the same conditions. to: maximize the finer; particles. screen analysis of: this material was:

Screen mesh number:

The Rotap Wt. percent retained A quantity of this 9600 r.p.m. material was placed in an 8'-0unce glass jar as described in run 1. After 2 days exposure to: room temperature of about 75 F. and a relative humidity between about 50 and 70%, the material was caked solid and adhered firmly to the glass jar. e, jar could be inverted without detaching any of the caked'material.

Run 4.About- 1 weight percent of agent (same as in run 2) was added to a quantity of-the 9600 r.p.m. material. Microscopic examination of the mixture showed the ammonium nitrate particles to be completely coated with a layer of the agent. After 5 months exposure to the same atmospheric conditions as those given in run 2 the coated ammonium nitrate was still free-flowing and quite free of lumps.

Example III 75 F. and a relative humidity of about 70-80%.

The Rotap screen analysis of this 14,000 r.p.m. material-made immediately after grinding-was:

Screen mesh number: Wt. percent retained +120 68 +200 18 +325 7 -325 7 Run 6.--Freshly ground 14,000 r.p.m. material was mixed with 2 weight percent of H 8 treated insoluble Prussian blue (same particle size as run 2) and stored in a loosely capped glass jar. Under the atmospheric conditions of run 2 this material was after 5 months exposure free-flowing and quite free of lumps.

Example IV Run 7.Under the same atmospheric conditions as those given in run 5, 50 grams of dynamite grade ammonium nitrate and 1 gram of H 8 treated insoluble Prussian blue were charged to the Mikro-Pulverizer operating at 14,000 r.p.m. The ammonium nitrate and the agent were comingled by shaking in a jar before being charged to the hopper. The total time for two passes through the pulverizer in this run was less than 5 minutes. The Rotap analysis of the coated ammonium nitrate product from this run was:

Wt. percent retained Screen mesh number:

+120 Trace +200 4 +325 32 325 64 This run illustrates the sensational reduction in grinding time obtained by the use of this agent as a grinding aid and the simultaneous sensational decrease in average particle size of the product ammonium nitrate. It is obvious that a considerable decrease in speed of the Mikro-Pulverizer can be made when using grinding aid in=order to obtain the sameparticle size distribution as that given in run 5.

Electron microscopic examination of the product ammoniumnitrate particles showed the. ammonium nitrate to be completely coated, with alayer of. agent. coated ammonium nitrate particles showed no evidence of agglomeration after about 5 months exposure to the conditions of run 2.

Example-V The ammonium nitrate is ground in. a controlled humidity room. The temperature in this room. is maintained at between about 65 and '85" F. (about the same as the outside atmosphere) and the relative; humidity is maintainedat between about 30and The relative humidity is maintained at these l'evels'because it is gen erally believed that. relative humidity is. about;

the point at which caking occursattemperatures .in the range of about to F. (Experience: here. shows this belief to be erroneous.)

However, in the absence of a grinding aid considerable difliculty Was had in obtaining adequate amounts of ground material owing to balling up in the hopper and blinding of the screens. Then the ground material could not be stored, i. e., it had to be used immediately to avoid any caking.

By the use of anticaking-agent, it has been possible to greatly decrease grinding time and to eliminate the grinding of small batches of ammonium nitrate just prior to use. Now, a single large batch is ground periodically, in the presence of about 2 weight percent of anticakingagent, and the ground material is stored until neededthereby effecting a large saving in time and manpower.

Thus having described the invention, what is claimed is:

l. Ammonium nitrate particles coated with a comminuted H 8 treated iron-ferrocyanide which particles are characterized by substantially free-flowability at at mospheric humidity and by a storage stability substantially that of dynamite grade ammonium nitrate, which H 3 treated iron-ferrocyanide is prepared by contacting iron-ferrocyanide with hydrogen sulfide gas at a temperature between about +20 and about +65 C. for a time sufiicient to produce a solid reaction product containing between about 2 and about 7 weight percent of sulfur.

2. The coated particles of claim 1 wherein said cyanide is Prussian blue.

3. The coated particles of claim 1 wherein said cyanide is insoluble Prussian blue.

4. A particulate composition consisting of ammonium nitrate particles coated with between about 0.1 and 5 weight percent, based on ammonium nitrate, of an agent, having a particle size between about 0.01 and 10 microns, where the smaller percentage corresponds to larger ammonium nitrate particles, and which composition is characterized by free-flowability at atmospheric humidity and by a storage stability substantially that of dynamite grade ammonium nitrate, which agent is prepared by treating an iron-ierrocyanide with hydrogen sulfide at between about +20 and +65 C. to produce a solid reaction product having a sulfur content between about 2 and 7 Weight percent is reached.

5. The composition of claim 4 wherein said cyanide is prepared from insoluble Prussian blue.

6. An improved method of grinding ammonium nitrate in the presence of atmospheric humidity, which method consists of grinding ammonium nitrate particles in the presence of an H S treated iron-ferrocyanide to produce particles smaller in size than the particles charged, which H 5 treated iron-ferrocyanide is prepared by contacting iron-ferrocyanide with hydrogen sulfide gas at a tempera. ture between about +20 and about +65 C. for a time suflicient to produce a solid reaction product containing between about 2 and about 7 weight percent of sulfur.

7. The method of claim 6 wherein the particle size of said cyanide is between about 0.01 and 10 microns.

The

8. Themethod of claim 6 wherein said cyanide is ferric ferrocyanide. p

9. The method of claim 6 wherein said cyanide is present in an amount at least suflicientto substantially coat the surface of the product ammonium nitrate particles.

10. A non-caking ammonium nitrate composition which consists of ammonium nitrate particles having a -Rotap screen analysis: +14 mesh, about 3 wt. percent; +30 mesh, about 80 wt. percent; +80 mesh, about 121wt. percent; and -80 mesh, about 5 wt. percent; coated with about 0.3 wt. percent, based on nitrate, of agent, having a particle size between about 0.01 and microns, mainly below about 1 micron, which agent has been prepared by treating insoluble Prussian blue with H S gas at a temperature between about C. and +65 C. until the solid reaction product attains a substantially constant sulfur content.

trate particles in the presence of atmospherichumidity,

which method consists of grinding. ammonium nitrate particles in the presence of betweenabout 0.1 and 5 weight percent, based on the nitrate, wherein the larger amount corresponds to the finer average particle size of the ammonium nitrate, of H treated iron-ferrocyanide, having a particle size between about 0.01 and 10 microns, which H S treated iron-ferrocyanide is prepared by contacting iron-ferrocyanide with hydrogen sulfide gas at a temperature between about +20 and about C. for a time suificient to produce a solid reaction product containing between about 2 and about 7 weight percent of sulfur. A l l 286,132 Harned Oct. 2, 18 83 

1. AMMONIUM NITRATE PARTICLES COATED WITH A COMMINUTED "H2S TREATED IRON-FERROCYANIDE" WHICH PARTICLES ARE CHARACTERIZED BY SUBSTANTIALLY FREE-FLOWABILITY AT ATMOSPHERIC HUMIDITY AND BY A STORAGE STABILITY SUBSTANTIALLY THAT OF DYNAMITE GRADE AMMONIUM NITRATE, WHICH "H2S TREATED IRON-FERROCYANIDE" IS PREPARED BY CONTACTING IRON-FERROCYANIDE WITH HYDROGEN SULFIDE GAS AT A TEMPERATURE BETWEEN ABOUT +20* AND ABOUT +65*C. FOR A TIME SUFFICIENT TO PRODUCE A SOLID REACTION PRODUCT CONTAINING BETWEEN ABOUT 2 AND ABOUT 7 WEIGHT PERCENT OF SULFUR. 